1. Field of the Invention
This invention relates to a method for the production of vinyl chloride polymers or copolymers. More particularly, the invention relates to a method of preparing porous, low molecular weight vinyl chloride polymer or copolymer resins. The instant resins are prepared by polymerizing vinyl halide monomers alone or in combination with comonomers copolymerizable therewith in the presence of a dual function porosifier/molecular weight modifier.
2. Background
Specialty Polyvinyl chloride (PVC) resins for use in applications such as injection molding and extrusion molding to form products that have thin walls, large surface areas, deep draw and/or intricate surface details requires resins that possess high plasticizer absorption capabilities and low molecular weights. Accordingly, porous, low molecular weight resins would be easier to fabricate into finished products because of their low melt viscosities, low shear sensitivity, good heat stability, superior plasticizer absorption, and excellent flow characteristics.
Low molecular weight PVC resins can be produced by polymerization at relatively high temperatures. Conventional high-temperature polymerization methods, however, are undesirable because of the extended gelation time of the resin, inferior flow of the resin in molding, and increased amounts of residual vinyl chloride monomer absorbed into the nascent resin particles during polymerization. Moreover, high temperatures, i.e., above about 65.degree. C., are generally detrimental to the formation of porous resins. In order to overcome these drawbacks, molecular weight modifiers (chain transfer agents) and porosifiers are used to control the molecular weights and porosities of the polymers, thereby forming resins having the desired physical properties.
A wide variety of compounds have been employed as molecular weight modifiers in the production of polyvinyl chloride resins. Among these, certain mercaptan containing compounds have been employed in several known methods to control the molecular weight of PVC resins. For example, U.S. Pat. No. 4,189,552 discloses the use of 2-mercaptoethanol as a molecular weight modifier in the suspension polymerization of vinyl chloride. The '552 patent teaches that the introduction of mercapto compounds into the reaction medium at a time when the monomer conversion is less than 1% produces adverse effects on particle characteristics. Because of the detrimental effects that mercapto compounds have on collodial stability, these compounds must be added portionwise (metering) and in low concentrations (under 0.03 part per 100 parts monomer) during the course of polymerization in order to prevent a course or solid charge.
To overcome this problem U.S. Pat. No. 4,797,458 discloses a method wherein the mercaptan is encapsulated in a non-polymerizable material which is miscible with the mercaptan and is substantially insoluble in water. Suitable non-polymerizable materials include dioctylphthalate, low molecular weight poly(caprolactones) and polysilicones, among others. However, this method has its drawbacks in that the molecular weight modifier must be mixed with the non-polymerizable material before addition into the reaction medium.
In a similar approach, U.S. Pat. No. 4,013,824 discloses the use of certain organosulphur molecular weight modifiers including mercaptothiazoles and 2-mercaptobenzothiazoles. Again, this system has its drawbacks in that the modifier must be mixed with an emulsifying additive before the onset of polymerization. Moreover, the disclosed process relates to an emulsion polymerization method wherein the molecular weight modifier is coagitated within an emulsifier and a hydrocarbon diluent prior to the initiation of the polymerization reaction.
U.S. Pat. Nos. 3,945,958; 3,951,925; and 4,695,616 all disclose attempts to produce vinyl chloride polymers and copolymers having improved porosity. However, the methods disclosed therein require extremely specific suspending agents, catalysts, additives, low polymerization temperatures, as well as strict control over process conditions.
Heretofore, attempts to make super porous, low molecular weight resins at high polymerization temperatures (i.e., above about 65.degree. C.) have been unsuccessful. Moreover, no attempt has been made to produce a super porous, low molecular weight resin by utilizing a dual function molecular weight/porosity modifier.
Accordingly, it would be highly desirable to provide a polymerization process that does not require specific temperature constraints and utilizes a dual function modifier without the need for special handling or additives to produce a super porous, low molecular weight resin.